1. FIELD OF THE INVENTION
The invention relates to a robust system to send digital data, such as tracking and telemetry information, through a contaminated environment, which may be noisy, unstable, and/or multipath, where multiple transmission paths, including fading paths, interfere with accurate message transmission. A further need is to minimize the message length while providing an increased ability to reject noise.
2. DESCRIPTION OF THE PRIOR ART
Several naturally occurring transmission media used as communications paths are subject to multiple transmission paths which fade in and out of existence at unpredictable intervals Among these are underwater paths and paths involving the ionosphere.
Numerous attempts have been made to achieve data transmission through contaminated environments, including quasi-random sequences of very short Phase Shift Keyed (PSK) signals, which are intended to get entirely through the transmission path before signals from longer alternate paths can begin to get through. Other signals called Spaced Frequency Shift Keyed (SFSK) are spaced apart in time sufficiently to allow the signals from the longer alternate paths to die out before the next short burst of signal is sent. These methods have one disadvantage that special effort must be made to match the signal length and interval to the available transmission paths, and another disadvantage that the waiting intervals involved with SFSK make transmission of a length message quite slow.
Chauncey S. Miller, in U.S. Pat. No. 3,810,019, used a multifrequency communication system in which each data symbol of the data symbol set being used was assigned to a subgroup of the available frequencies, in accordance with a Hadamard matrix. This was an interesting attempt to solve the problem, but his attempts to manipulate the Hadamard matrix, particularly in replacing the minus ones with zeroes, destroyed its natural orthogonality, thereby, at that point, removing it from the scope of the definition of a Hadamard matrix. The utility of the Miller system did not extend to cases where there was a frequency shift in the transmission path, such as occur with Doppler changes.